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A decrease of BC from .400 to .350 will increase the drop and wind deflection of the
bullet. BUT, not proportionately; since reducing BC from .400 to .350 is a reduction of 12.5%,
and: 200 Yd Drop increases 2%
200 Yd. Deflection increases 15%
1000 Yd. Drop increases 6%
1000 Yd. Deflection increases 11%
The point is that small changes in the BC yield some smaller changes in bullet behavior, and
time spent on assuring that the newly designed bullet has the identical BC as the original bullet is
better spent in other pursuits-perhaps developing a bullet lube recipe containing yak butter.
Note also that some of the above work is based on an article written by me for the ASSRA News
in the distant past, and that I learned most of the hard stuff (that homologue business) from
articles written by others in The Fouling Shot including W. C. Davis. The only things original
with me are the observation that BC is approximately constant, for similar-shaped bullets of the
same length, regardless of caliber; and that fascinating table of weights of cylinders of different
alloys.
BALLISTIC COEFFICIENTS (BC)
The BC of a bullet is a measure of how efficiently that bullet goes through the air; of how
much velocity is lost as the bullet travels down range.
BC's are measured in decimals such as, 269 or .361. The higher the number, the higher the BC,
the less the effect of wind on the bullet and the higher the retained velocity of the bullet.
The BC of a bullet varies with muzzle velocity, air density, wobble or yaw, and the time of high
tide. I believe that three digits to the right of the decimal is one too many, and that BC should
more properly and accurately be recorded as .27 or .36. The path of the bullet does not vary
proportionately with variations in BC. If two bullets are fired at the same muzzle velocity, and if
one has a higher retained velocity at (say) 200 yards, then the bullet with the higher retained
velocity has a higher BC.
Bullets with the same BC, fired at the same muzzle velocity, follow the same path from
muzzle to target. So, for instance, if we fire 22 caliber and 45 caliber bullets with the same BC
and muzzle velocity, in the same conditions; then the trajectories of the bullets will be the same,
and they will both be affected by the wind to the same extent. The higher the BC, the less the
bullet is displaced by the wind, at any given muzzle velocity.
The BC of a bullet is a function of the form (shape) of the bullet, and of its sectional
density (SD). Sectional Density is Weight/Caliber Squared, units are pounds per square inch.
Bullets with the same form and the same SD have the same BC.
The effect of form on BC can be thought of as having three parts. The form component is
determined by the form of the nose, the form of the base, and the form and length of the side.
The form of the nose and the base are very important determinants of BC, but the form and
length of the side is not an important determinant of BC.
About wind drift and velocity.
BC, bullet velocity and wind speed determine how much a bullet is deflected by the
wind on the way to the target.
BC and bullet velocity don't matter when you're shooting in the calm, with no wind. I
prefer shooting when there's no wind, although the wind, especially a gusting wind, is a
wonderful excuse.
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